99.9% of people having this problem have also forgotten promotions or genius discounts on their OTAs such as Booking.com, Expedia or AirBnB which have independent minimum stay restrictions (non-connected with OTA Sync).
Check if you currently have certain promotions set on the sales channel, if a special package is active for you, or if the reservation was entered before you made the minimum stay change.
Also you should check under which rateplan reservation was booked and check-in Setting > Pricing Plans > Specific Pricing Plan Settings which Restriction plan is mapped under it and then check for that Restriction plan the minimum number of nights for booked period.